CT2015

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CarbonTracker CT2015

CarbonTracker is a CO2 measurement and modeling system developed
by NOAA to keep track of sources (emissions to the atmosphere) and sinks (removal from the
atmosphere) of carbon dioxide around the world. CarbonTracker uses atmospheric
CO2 observations from a host of collaborators and simulated atmospheric transport to estimate these
surface fluxes of CO2. The current release of CarbonTracker,
CT2015, provides global estimates of surface-atmosphere fluxes of
CO2 from January 2000 through December 2014.

North America is a source of CO2 to the atmosphere. The
natural uptake of CO2 that occurs mostly east of the
Rocky Mountains removes about a third of the CO2
released by the use of fossil fuels. [read
more]

CarbonTracker CO2 weather for June-July, 2008.
Warm colors show high atmospheric CO2 concentrations, and
cool colors show low concentrations. As the summer growing season takes hold,
photosynthesis by forests and crops draws concentrations of
CO2 down, opposing the general increase from fossil fuel
burning. The resulting high- and low-CO2 air masses are
then moved around by weather systems to form the patterns shown
here. [More on CO2 weather]

Global CO2 budget

From 2001 through 2014, CO2 emissions to the atmosphere from
burning of fossil fuels in CT2015 rose from
6.8 PgC yr-1 to 9.8 PgC yr-1 (1 petagram of
carbon is 1015 gC, or 1 billion metric tons C, or 3.67 billion metric tons
CO2). Global fossil fuel emissions have increased steadily year
upon year, with the exception of 2008 and 2009 when emissions held nearly constant
following the global economic recession (Figure 1). Fossil fuel emissions are concentrated
in areas with high population density and economic activity, and emissions inventory
information used in CT2015 indicates that 82% of fossil fuel emissions come
from the industrialized northern extratropics.

Figure 1. Annual global emissions. The bars in this figure represent
CO2 emissions for each year in PgC yr-1
over the globe. CarbonTracker models four types of surface-to-atmosphere exchange of
CO2, each of which is shown in a different color: fossil
fuel emissions (tan), terrestrial biosphere flux
excluding fires (green), direct emissions from fires
(red), and air-sea gas exchange (blue). Negative emissions indicate that the flux removes
CO2 from the atmosphere. The net surface exchange, computed
as the sum of these four components, is shown as a thick black line.
[Explore these data
further]

The other major source of CO2 is wildfires, which in CT2015 add an additional 1.7-2.1 PgC yr-1 to the atmosphere. In
contrast to fossil fuel emissions, wildfire CO2 comes principally
from tropical and southern land. 84% of wildfire emissions in CT2015 are
in those regions.

Offsetting these sources are natural sinks on land and in the ocean. Together, these
sinks absorb about half the anthropogenic CO2 emitted into the
atmosphere. Over the period 2001-2014, the CT2015 global sum of "natural"
fluxes (fire emissions, the land biosphere sink, and the ocean sink) is 49.7% of fossil
fuel emissions over the same time period. The atmospheric CO2
growth rate would be about twice the observed rate without these sinks. CarbonTracker is
designed to identify these sinks in order to better understand the mechanisms behind them.

According to CT2015, the world's oceans absorb 1.8 to
2.9 PgC yr-1. This natural sink exists as a direct result of
increasing atmospheric CO2 concentrations, as dissolved carbon
concentrations in the ocean increase to reach equilibrium with the atmosphere. However,
the large-scale circulation of the ocean and biological, physical, and chemical carbon
cycling cause there to be a source of carbon to the atmosphere in the tropics. In
CT2015, this natural source of between 0.5 and
0.8 PgC yr-1 in the tropics is offset by large extratropical sinks of
2.6-3.4 PgC yr-1.

The terrestrial biosphere is also a net sink of CO2, due mainly
to two processes. These are CO2 fertilization, in which plants
grow faster since they can more easily acquire carbon dioxide for photosynthesis, and the
effects of human land-use practices, including fertilization, irrigation, fire suppression,
and recovery from past land use. CarbonTracker attempts to locate these land sinks
spatially and temporally, so hypotheses about their mechanisms can be tested. CT2015 finds widely-scattered terrestrial CO2 sinks, with
significant absorption of carbon dioxide by northern temperate and boreal regions
(1.5-2.9 PgC yr-1, about 63% of the global land total of
2.4-4.9 PgC yr-1 ).

In this text, fluxes reported are reported as ranges when possible, to provide some
context for how well CarbonTracker constrains the long-term average flux. These ranges are
computed as the minimum and maximum values from the sequence of annual CT2015 flux estimates. A large range indicates a high degree of interannual
variability in the flux estimate.

CO2 sources and sinks over North America

CT2015 results indicate that North America ecosystems have been a net
sink of 0.6 ± 1.3 PgC yr-1 over the period 2001-2014. This
natural sink offsets about one-third of the emissions of about 1.8
PgC yr-1 from the burning of fossil fuels in the U.S.A., Canada and
Mexico combined.

Figure 2. Drought and land sinks over
N. America.Top
Panel:United
States Drought Monitor percent area of the
continental U.S. undergoing different levels of
drought intensity. Bottom Panel: Annual land
sink estimates (including fire emissions) from
CT2015 for temperate North America.

Whereas fossil emissions are generally steady over this period, ranging between 1.7 and
1.8 PgC yr-1, the amount of CO2 taken up by
the North American biosphere varies significantly from year to year (Figure 2, bottom
panel). In terrestrial biosphere models, inter-annual variability in land uptake can be
related to anomalies in large-scale temperature and precipitation patterns. While the
CarbonTracker period of analysis is relatively short compared to the dynamics of
slowly-changing pools of biospheric carbon, episodes of extremes in net ecosystem exchange
(NEE) have been associated with climatic anomalies (see
e.g. Peters et al., 2007). CT2015 annual estimates of the
land sink over temperate N. America are clearly related to continental-scale drought
intensity (Figure 2). It is interesting to note that the inferred year-to-year variabilty
(the "range") of land uptake is actually as big as the average sink itself.

Widespread droughts in the U.S. west and Canada during 2002 and 2012 resulted in
relatively small annual uptake by terrestrial ecosystems in temperate North America (Figure
2). In these years, land ecosystems accounted for a sink of only about
0.2 PgC yr-1. This is about half the sink of an "average" year, in
which these same land ecosystems remove about 0.4 PgC yr-1.

Spatial distribution of North American surface fluxes

CarbonTracker flux estimates include sub-continental patterns of sources and sinks
coupled to the distribution of dominant ecosystem types across the continent (Figure 3). We
have greater confidence in countrywide totals than in estimates of regional sources and
sinks, but we expect that such finer-scale estimates will become more robust with future
expansion of the CO2 observing nework. Our results indicate that
the sinks are mainly located in the agricultural regions of the U.S. and Canadian midwest,
and boreal forests in Canada.

Figure 3. Average ecosystem fluxes. The pattern of net ecosystem exchange (NEE)
of CO2 of the land biosphere averaged over 2001-2014, as
estimated by CarbonTracker CT2015. This NEE represents
land-to-atmosphere carbon exchange from photosynthesis and respiration in
terrestrial ecosystems, and a contribution from fires. It does not include fossil
fuel emissions. Negative fluxes (blue colors) represent CO2
uptake by the land biosphere, whereas positive fluxes (red colors) indicate regions
in which the land biosphere is a net source of CO2 to the
atmosphere. Units are gC m-2 yr-1.

Word of caution about high-resolution biological flux maps Figure 3 shows estimated
fluxes by ecoregion. While we also
provide flux maps and data with a finer 1° x 1° spatial resolution, for
example on our
flux maps pages, these ecoregions define the actual scales at
which CarbonTracker operates. With the present observing network, the detailed one-degree
fluxes should not be interpreted as quantitatively meaningful for each block. Any
within-ecoregion patterns come directly from results of the
terrestrial biosphere model. Part of
this high-resolution patterning comes from variations of temperature, precipitation, light,
plant species, and soil type over each ecoregion. To spread the influence of measurements
from the sparse observation network, CarbonTracker makes adjustments uniformly over an
entire ecoregion. These adjustments
scale the net ecosystem flux of CO2 predicted by the terrestrial
biosphere model by the same factor across each ecoregion. Thus we caution that the
1° x 1° spatial detail in CarbonTracker land fluxes is based on the
simulations of the terrestrial biosphere model and the assumption of large-scale ecosystem
coherence. This has not been verified by observations.

The CarbonTracker observing system

CarbonTracker surface flux estimates are optimally consistent with 557,486 atmospheric
CO2 observations from the GLOBALVIEWplus-1.0 ObsPack,
comprising 298 time series datasets from around the world using a variety of measurement
techniques and platforms (Table 1, Figure 4). These observation are contributed
by collaborators from 32 different laboratories.
CO2 observational data can be accessed by downloading
the GLOBALVIEWplus-1.0
ObsPack, or if modeled observations are also required, the CT2015 ObsPack. More information on
CO2 measurements used in CT2015 can be found in
the observations documentation.

Observation

Number of

Number of

Not For

Assimilation Observations

Type

Datasets

Observations

Assimilation

Total

Accepted

Rejected

surface-insitu

70

2 778 083

2 364 813

413 270

411 830

1 440

surface-flask

132

41 533

2 059

39 474

39 140

334

surface-pfp

22

18 997

8 251

10 746

10 692

54

tower-insitu

24

1 386 634

1 294 982

91 652

90 828

824

aircraft-pfp

41

52 618

52 618

0

0

0

aircraft-flask

3

4 472

4 472

0

0

0

aircraft-insitu

2

427 034

427 034

0

0

0

shipboard-flask

2

2 344

0

2 344

2 315

29

Total

298

4 711 715

4 154 229

557 486

554 805

2 681

Table 1. CarbonTracker
CT2015 observations by observation type, which
comprises measurement platform and instrument
(in ObsPack
parlance this is a "project"). More information on
CO2 measurements used in CT2015 can be found in
the observations
documentation.

Figure
4. CarbonTracker Observational Network Click on any site
marker for more information. Double-click on a site marker to
center the map on that site.

Calculated time-dependent CO2 fields throughout the global
atmosphere

A "byproduct" of the data assimilation system, once sources and sinks have been
estimated, is that the
mole fraction of CO2 is
calculated everywhere in the model domain and over the entire 2000-2014 time period, based
on the optimized source and sink estimates (Figure 1). As a check on model transport
properties and CarbonTracker inversion performance, calculated CO2
mole fractions are regularly compared with ~484,000 measurements from
46 aircraft datasets taken by NOAA/ESRL and collaborators. These independent samples
are not used to estimate fluxes in CarbonTracker, but rather set aside for cross-validation.

Since CarbonTracker simulates CO2 throughout the entire
atmospheric column, the model atmosphere can be sampled like satellite
(GOSAT
and OCO-2) and ground-based remote sensing instrument
(TCCON) retrievals of
CO2. Examples of our agreement with the latter can be found on
our TCCON page.

Flux uncertainties

Figure 5. Carbon dioxide weather Shown is the daily average of the
pressure-weighted average mole fraction of carbon dioxide in the free troposphere as
modeled by CarbonTracker for March 20, 2009. Units are micromoles of
CO2 per mole of dry air (μmol mol-1), and the
values are given by the color scale depicted under the graphic. The "free
troposphere" in this case is levels 6 through 10 of the TM5 model. This corresponds
to about 1.2km above the ground to about 5.5km above ground, or in pressure terms,
about 850 hPa to about 500 hPa. Gradients in CO2
concentration in this layer are due to exchange between the atmosphere and the earth
surface, including fossil fuel emissions, air-sea exchange, and the photosynthesis,
respiration, and wildfire emissions of the terrestrial biosphere. These gradients
are subsequently transported by weather systems, even as they are gradually erased
by atmospheric mixing.

It is important to note that at this time the uncertainty estimates for CarbonTracker
sources and sinks are themselves quite uncertain. They have been derived from the
mathematics of the ensemble data assimilation system, which requires several educated guesses for initial uncertainty
estimates. The paper describing CarbonTracker (Peters et al. (2007), Proc. Nat. Acad. Sci. vol. 104, p. 18925-18930)
presents different uncertainty estimates based on the sensitivity of the results to 14
alternative yet plausible ways to construct the CarbonTracker system. For example, the 14
realizations produce a range of the net annual average terrestrial emissions in North America
of -0.40 to -1.01 PgC -1 (negative emissions indicate a sink). The
procedure is described in the Supporting Information Appendix to that paper, which is freely
downloadable from the PNAS web site.

Furthermore, the estimates do not take into account several additional factors noted
below. The calculation is set up for sources and sinks to slowly revert, in the absence of
observational data, to first guesses of net ecosystem exchange, which are close to zero on
an annual basis. This set-up may result in a bias. Also due to the sparseness of
measurements, we have had to assume coherence of ecosystem processes over large distances,
giving existing observations perhaps an undue amount of weight. The process model for
terrestrial photosynthesis and respiration was very basic, and will likely be greatly
improved in future releases of CarbonTracker. Easily the largest single annual average source
of CO2 is emissions from fossil fuel burning, which are currently
not estimated by CarbonTracker. We use estimates from emissions inventories (economic
accounting) and subtract the CO2 mole fraction signatures of those
fluxes from observations. As a result, the biosphere and ocean fluxes estimated by
CarbonTracker inherit error from the assumed fossil fuel emissions. While these emissions
inventories may have a small relative error on global scales (perhaps 5 or 10%), any such
bias translates into a larger relative error in the annual average ecosystem sources and sinks,
since those fluxes have smaller magnitudes. We expect to add a process model of fossil fuel
combustion in future releases of CarbonTracker. Finally, additional measurement sites are
expected to lead to the greatest improvements, especially to more robust and specific
source/sink results at smaller spatial scales.

Consistency of modeled and observed atmospheric CO2 growth
rates

Global atmospheric CO2growth rates inferred directly from
observed carbon dioxide at marine surface sites are consistent with those modeled by
CarbonTracker, both in their average values and in their year-to-year variations (Figure
6). These global growth rates continue to hover at around
4 PgC yr-1, or around
1.9 ppm yr-1 (Figure 6).

Figure 6. Atmospheric CO2 growth rates. Observed
atmospheric CO2 growth rates (source: NOAA ESRL page on global
trends in CO2) are plotted against the atmospheric
CO2 growth rate inferred
from CT2015 global fluxes. Note that error bars on the observed growth rates are relatively
small and may not be visible on this plot.